1. Field of the Invention
This invention relates to an improved swirl vane separator for drying steam. It is particularly applicable to the swirl vane separators used in the secondary sides of nuclear steam generators.
2. Description of the Prior Art
Swirl vane separators for drying steam are well known in the prior art. In nuclear steam generators, a bank of between twelve and sixteen of such separators are frequently mounted at the upper end of the secondary side of the generator in order to remove the water droplets entrained in the steam. The removal of these droplets is important for two reasons. First, since the steam produced by such nuclear steam generators is ultimately directed against the turbine of an electric generator at pressures ranging between 900 and 1000 pounds per square inch, any residual water droplets in the steam can cause a significant amount of errosion in the blades of the turbine. Secondly, the water losses that occur as a result of such wet steam increase the amount of water that must be supplied to the steam generator, which in turn accelerates the creation of sludge deposits in the generator. Since such sludge deposits are responsible for much of the corrosion that attacks the heat exchange tubes in the steam generator, it is desirable that such water losses through the steam be reduced as much as possible.
No practical device has yet been developed which can remove all of the water droplets from such steam. However, a primary bank of swirl vane separators, in combination with an in-tandem secondary bank of vane-type separators, are capable of reducing the moisture droplet content of the steam produced by the generator to an amount equal to less than one-fourth of 1% of the total steam weight. While such a moisture droplet content does not completely prevent turbine blade erosion, it reduces the amount of erosion which occurs over time to a commercially acceptable rate. Additionally, the sludges created as a result of the associated water losses are small enough to be dealt with by known maintenance procedures.
Even though the primary and secondary banks of the water separators used in the secondary sides of nuclear steam generators have often proven equal to the task of reducing the moisture content of the resulting steam to within a commercially acceptable limit, difficulties can sometimes arise if the internal configuration of these swirl vane separators is not precisely matched to the particular characteristics of the steam which flows through the housing of the separator. But before one can appreciate the nature of these difficulties, some understanding of the structure and function of such swirl vane separators is necessary.
Swirl vane separators generally include a housing having a steam inlet at one end, a steam outlet at the other end, and a set of pitched blades mounted therebetween. The housing, in turn, is formed from a cylindrical riser barrel that is concentrically disposed within a cylindrical downcomer barrel. The outlet end of the downcomer barrel is covered by a plate-like cap. The steam outlet of the housing is formed from a round aperture that is located in the center of the cap. To minimize the pressure drop that the flow of steam experiences across the separator, the diameter of the steam outlet is frequently about three-quarters of the diameter of the cap which covers the downcomer barrel. A pitched set of stationary blades is mounted in the riser barrel for providing a radial component of motion to the water droplets in the wet steam as they flow from the inlet of the housing to the steam outlet. These blades are typically inclined to the horizontal somewhere between 30 and 37 degrees. In the interior of the housing, a downcomer opening is provided for receiving droplets of water which are flung against the sides of the riser barrel as a result of the radial component of motion imparted by the pitched blades. This downcomer opening is formed by spacing the riser barrel a short distance from the cap which overlies the outlet end of the downcomer barrel, which forms an annular gap between this cap and the upper edge of the riser barrel. The downcomer opening communicates with a downcomer flowpath that is defined in the annular space between the concentrically disposed downcomer and riser barrels.
In operation, the inlet end of the separator housing is submerged in the water boiled by the bundle of heat exchange tubes present in the secondary side of the generator. Inside the housing, the pitched set of blades is positioned at a point somewhere above the boiling water. The flow of wet steam produced by this boiling water rises, and flows through the pitched blades secured by the riser barrel of the housing. The pitch of the blades generates a radial component of motion in both the flow of steam and the water droplets entrained therein, and causes both to adopt a helical pattern of movement. However, the greater mass of the water droplets entrained in the steam causes them to have relatively wider spiral trajectories than the surrounding steam, which ultimately causes them to impinge at some point against the inner wall of the riser barrel.
If the separator has been properly tuned to match the specific characteristics of the steam which flows therethrough, a substantial amount of the droplets of water entrained in the steam will be flung into the annular downcomer opening defined in the gap between the upper edge of the riser barrel, and the inner surface of the cap which overlies the downcomer barrel. The resulting accumulation of water will then flow down the downcomer flowpath defined between the downcomer and riser barrels. By contrast, if the separator is not properly tuned to these characteristics, many of these droplets of water will either collide against the inner wall of the riser barrel, and become re-entrained, or will simply flow out of the steam outlet of the housing. In either case, the drying effectiveness of the swirl vane separator is significantly reduced under such mistuned conditions.
One previous method for avoiding such mis-tuned conditions involved the use of swirl vane separators having relatively small diameters (i.e., ten- vs. twenty-inch diameters) and whose internal geometry (i.e., the pitch and position of the blades) was optimized by means of a flow of freon that stimulated the average properties of the wet steam produced across the plenum of the generator. Unfortunately, this particular method has not proven to be entirely satisfactory. The utilization of a relatively larger number of smaller swirl vane separators increases the cost of the primary bank of water separators used in the secondary side of the steam generator, and mechanically complicates the structure of the steam generator as a whole. The applicant has also discovered that the freon steam flow simulation has not, in practice, resulted in optimally tuned separators for two reasons. First, the physical properties of freon are significantly different from the characteristics of water. Secondly, and more importantly, it has been found that the characteristics of the steam produced in various regions above the tube bundle of the steam generator differ substantially across the plenum. For example, the steam produced above the "hot leg" section of the secondary side has a substantially higher flow rate than the steam produced over the "cold leg" side. Hence, any approach that attempted to optimize the blade configurations of all the separators in the primary bank in accordance with a simulated "average" flow was inherently erroneous since what constituted an optimal blade configuration varied substantially for different regions above the tube bundle.
Clearly, there is a need for an individually tunable swirl vane separator whose internal geometry may be easily matched to the specific characteristics of the steam that specifically flows through the separator. Ideally, such a separator should also be easily adjustable through inexpensive mechanisms that are compatible with existing swirl vane separators. Such a separator should have a housing whose diameter is large enough so that only a moderate number of such separators are necessary for the primary bank of a steam generator in order to minimize the overall complexity and cost that the separators impose on the steam generator design.